TMR device with novel free layer structure

Abstract

A TMR sensor that includes a free layer having at least one B-containing (BC) layer made of CoFeB, CoFeBM, CoB, CoBM, or CoBLM, and a plurality of non-B containing (NBC) layers made of CoFe, CoFeM, or CoFeLM is disclosed where L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb. One embodiment is represented by (NBC/BC).sub.n where n2. A second embodiment is represented by (NBC/BC).sub.n/NBC where n1. In every embodiment, a NBC layer contacts the tunnel barrier and NBC layers each with a thickness from 2 to 8 Angstroms are formed in alternating fashion with one or more BC layers each 10 to 80 Angstroms thick. Total free layer thickness is <100 Angstroms. The free layer configuration described herein enables a significant noise reduction (SNR enhancement) while realizing a high TMR ratio, low magnetostriction, low RA, and low Hc values.

Claims

1. A magnetoresistive element in a magnetic device, comprising: (a) a SyAP pinned layer; (b) a free layer consisting of (CoFeM/CoFeB).sub.n/CoFeM, (CoFe/CoFeBM).sub.n/CoFe, (CoFeM/CoFeBM).sub.n/CoFeM, (CoFeLM/CoFeB).sub.n/CoFeLM, or (CoFeLM/CoFeBM).sub.n/CoFeLM wherein n1, L is one of Ta, Ti, W, Zr, Hf, Tb, or Nb, M is one of Ta, Ti, W, Hf, or Tb, and L is unequal to M, and each of said CoFeB or CoFeBM layers has a greater thickness than each of said CoFe, CoFeM, or CoFeLM layers, and; (c) a tunnel barrier layer having a first surface that contacts the SyAP pinned layer and a second surface that contacts a CoFe, CoFeM, or CoFeLM layer in said free layer wherein said second surface is opposite said first surface.

2. The magnetoresistive element of claim 1 wherein the boron content in each of said CoFeB layers is between about 10 and 40 atomic % and the boron content in each of said CoFeBM layers is between about 5 and 40 atomic %.

3. The magnetoresistive element of claim 1 wherein each of said CoFe, CoFeM, or CoFeLM layers has a thickness between about 2 and 8 Angstroms and each of said CoFeB or CoFeBM layers has a thickness from about 10 to 80 Angstroms.

4. The magnetoresistive element of claim 1 wherein the M content in said CoFeM alloy and the L+M content in said CoFeLM alloy is less than about 10 atomic %.

5. A magnetoresistive element in a magnetic device, comprising: (a) a SyAP pinned layer; (b) a free layer consisting of (CoFeM/CoB).sub.n, (CoFeLM/CoB).sub.n, (CoFe/CoBM).sub.n, (CoFeM/CoBM).sub.n, (CoFeLM/CoBM).sub.n, (CoFe/CoBLM).sub.n, (CoFeM/CoBLM).sub.n, or (CoFeLM/CoBLM).sub.n wherein n2, L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb, and L is unequal to M, and each of said CoB, CoBM, or CoBLM layers has a greater thickness than each of said CoFe, CoFeM, or CoFeLM layers, and; (c) a tunnel barrier layer having a first surface that contacts the SyAP pinned layer and a second surface that contacts a CoFe, CoFeM, or CoFeLM layer in said free layer wherein said second surface is opposite said first surface.

6. The magnetoresistive element of claim 5 wherein the boron content in each of said CoB layers is between about 5 and 30 atomic %.

7. The magnetoresistive element of claim 5 wherein each of said CoFe, CoFeM, or CoFeLM layers has a thickness between about 2 and 8 Angstroms and each of said CoB, CoBM, or CoBLM layers has a thickness from about 10 to 80 Angstroms.

8. The magnetoresistive element of claim 5 wherein the tunnel barrier layer is comprised of MgO, MgZnO, ZnO, Al.sub.2O.sub.3, TiOx, AlTiO, HfOx, ZrOx, or a combination of two or more of the aforementioned materials.

9. A magnetoresistive element in a magnetic device, comprising: (a) a SyAP pinned layer; (b) a free layer consisting of (CoFeM/CoB).sub.n/CoFeM, (CoFeLM/CoB).sub.n/CoFeLM, (CoFe/CoBM).sub.n/CoFe, (CoFeM/CoBM).sub.n/CoFeM, (CoFeLM/CoBM).sub.n/CoFeLM, (CoFe/CoBLM).sub.n/CoFe, (CoFeM/CoBLM)n/CoFeM, or (CoFeLM/CoBLM).sub.n/CoFeLM wherein n1, L is one of Ta, Ti, W, Zr, Hf, Tb, or Nb, M is one of Ta, Ti, W, Hf, or Tb, and L is unequal to M, and each of said CoB, CoBM, or CoBLM layers has a greater thickness than each of said CoFe, CoFeM, or CoFeLM layers, and; (c) a tunnel barrier layer having a first surface that contacts the SyAP pinned layer and a second surface that contacts a CoFe, CoFeM, or CoFeLM layer in said free layer wherein said second surface is opposite said first surface.

10. The magnetoresistive element of claim 9 wherein the boron content in each of said CoB layers is between about 5 and 30 atomic %.

11. The magnetoresistive element of claim 9 wherein each of said CoFe, CoFeM, or CoFeLM layers has a thickness between about 2 and 8 Angstroms and each of said CoB, CoBM, or CoBLM layers has a thickness from about 10 to 80 Angstroms.

12. The magnetoresistive element of claim 9 wherein the M content in said CoFeM alloy and the L+M content in said CoFeLM alloy is less than about 10 atomic %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view showing a TMR stack of layers according to one embodiment of the present invention.

(2) FIG. 2 is an enlarged drawing of the free layer in FIG. 1 that illustrates the various layers within the composite structure according to one embodiment of the present invention.

(3) FIG. 3 is an enlargement of the free layer in FIG. 1 that shows the layers within the composite structure according to a second embodiment of the present invention.

(4) FIG. 4 is a cross-sectional view showing a TMR stack of layers that has been patterned to form a MTJ element during an intermediate step of fabricating the TMR sensor according to one embodiment of the present invention.

(5) FIG. 5 is a cross-sectional view of a TMR read head having a MTJ element interposed between a top shield and bottom shield and formed according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) The present invention is a high performance TMR sensor featuring a composite free layer comprised of CoFeB, CoB, or alloys thereof in which excessive noise normally associated with these boron containing magnetic materials is reduced by forming a free layer configuration that includes a plurality of CoFe or CoFe alloy layers. While the exemplary embodiment depicts a TMR sensor in a read head, the present invention may be employed in other devices based on a tunneling magnetoresistive element such as CIP-GMR or CPP-GMR sensors. Although a bottom spin valve structure is described for a TMR sensor, the present invention also encompasses a top spin valve or multilayer spin valve configuration as appreciated by those skilled in the art. Drawings are provided by way of example and are not intended to limit the scope of the invention. For example, various elements are not necessarily drawn to scale and their relative sizes may differ compared with those in an actual device.

(7) Previously, the inventors have practiced a method in related U.S. Pat. No. 9,021,685 whereby a two step annealing procedure is used to restore some of the magnetic softness lost when a CoFeB layer is included in a free layer. The procedure involves applying a magnetic field with a first temperature and for a first length of time and then applying a magnetic field with a second temperature greater than the first temperature but for less than the first length of time. However, noise reduction is not achieved by this procedure.

(8) It should be understood that in a CoFeB based free layer, a certain B content is needed to achieve the required magnetic softness. However, a large amount of non-magnetic B (typically 20 atomic %) that increases softness also tends to introduce extra noise and cause a degradation in the signal-to-noise ratio (SNR) in the read head. Higher B content also leads to high magnetostriction () in the device which is a concern. Related U.S. Pat. No. 8,472,151 discloses how a CoFeB composition may be adjusted to reduce and how CoB with a slightly negative value may be used to replace CoFeB with a large positive value. However, there is still a significant noise associated with a CoB layer because of its boron content. Therefore, we were motivated to further modify the free layer to improve performance and have discovered a composite free layer structure containing boron that reduces noise (improves SNR) while realizing a high dR/R, low value, low RA, and low coercivity.

(9) Referring to FIG. 1, a portion of a partially formed TMR sensor 1 of the present invention is shown from the plane of an air bearing surface (ABS). There is a substrate 10 that in one embodiment is a bottom lead otherwise known as a bottom shield (S1) which may be a NiFe layer about 2 microns thick that is formed by a conventional method on a substructure (not shown). It should be understood that the substructure may be comprised of a wafer made of AlTiC, for example.

(10) A TMR stack is formed on the substrate 10 and in the exemplary embodiment has a bottom spin valve configuration wherein a seed layer 14, AFM layer 15, pinned layer 16, tunnel barrier layer 17, free layer 18, and capping layer 19 are sequentially formed on the substrate. The seed layer 14 is preferably a Ta/Ru composite but Ta, Ta/NiCr, Ta/Cu, Ta/Cr or other seed layer configurations may be employed, instead. The seed layer 14 serves to promote a smooth and uniform grain structure in overlying layers. Above the seed layer 14 is an AFM layer 15 used to pin the magnetization direction of the overlying pinned layer 16, and in particular, the outer portion or AP2 layer (not shown). The AFM layer 15 has a thickness from 40 to 300 Angstroms and is preferably comprised of IrMn. However, one of PtMn, NiMn, OsMn, RuMn, RhMn, PdMn, RuRhMn, or MnPtPd may be employed as the AFM layer.

(11) The pinned layer 16 preferably has a synthetic anti-parallel (SyAP) configuration represented by AP2/Ru/AP1 where a coupling layer made of Ru, Rh, or Ir, for example, is sandwiched between an AP2 layer and an AP1 layer (not shown). The AP2 layer which is also referred to as the outer pinned layer contacts the AFM layer 15 and may be made of CoFe with a composition of about 10 atomic % Fe and with a thickness of about 10 to 50 Angstroms. The magnetic moment of the AP2 layer is pinned in a direction anti-parallel to the magnetic moment of the AP1 layer. For example, the AP2 layer may have a magnetic moment oriented along the +x direction while the AP1 layer has a magnetic moment in the x direction. A slight difference in thickness between the AP2 and AP1 layers produces a small net magnetic moment for the pinned layer 16 along the easy axis direction of the TMR sensor to be patterned in a later step. Exchange coupling between the AP2 layer and the AP1 layer is facilitated by a coupling layer that is preferably comprised of Ru with a thickness from 3 to 9 Angstroms. The AP1 layer is also referred to as the inner pinned layer and may be a single layer or a composite layer. In one aspect, the AP1 layer is amorphous in order to provide a more uniform surface on which to form the tunnel barrier layer 17. The AP1 layer may be comprised of CoFeB, CoFe, or a composite thereof, and has an upper surface that contacts the tunnel barrier layer 17.

(12) In the exemplary embodiment that features a bottom spin valve configuration, the tunnel barrier layer 17 is preferably comprised of MgO because a MgO tunnel barrier is known to provide a higher TMR ratio than a TMR stack made with an Al.sub.2O.sub.3 or TiOx tunnel barrier. However, the present invention anticipates that the TMR stack may have a tunnel barrier made of MgZnO, ZnO, Al.sub.2O.sub.3, TiOx, AlTiO, HfOx, ZrOx, or combinations of two or more of the aforementioned materials including MgO.

(13) In an embodiment where the tunnel barrier layer 17 is made of MgO, a preferred process is to DC sputter deposit a first Mg layer having a thickness between 4 and 14 Angstroms on the pinned layer 16, and then oxidize the Mg layer with a natural oxidation (NOX) process before depositing a second Mg layer with a thickness of 2 to 8 Angstroms on the oxidized first Mg layer as described in related U.S. Pat. No. 8,472,151. In one aspect, the tunnel barrier is considered as having a MgO/Mg configuration. The second Mg layer serves to protect the subsequently deposited free layer from oxidation. It is believed that excessive oxygen accumulates at the top surface of the MgO layer as a result of the NOX process and this oxygen can oxidize a free layer that is formed directly on the MgO portion of the tunnel barrier layer. Note that the RA and MR ratio for the TMR sensor may be adjusted by varying the thickness of the two Mg layers in tunnel barrier layer 17 and by varying the natural oxidation time and pressure. Longer oxidation time and/or higher oxygen pressure will form a thicker MgO layer and increase the RA value.

(14) All layers in the TMR stack may be deposited in a DC sputtering chamber of a sputtering system such as an Anelva C-7100 sputter deposition system that includes ultra high vacuum DC magnetron sputter chambers with multiple targets and at least one oxidation chamber. Typically, the sputter deposition process involves an argon sputter gas and a base pressure between 510.sup.8 and 510.sup.9 torr. A lower pressure enables more uniform films to be deposited.

(15) The NOX process may be performed in an oxidation chamber within the sputter deposition system by applying an oxygen pressure of 10.sup.6 Torr to 1 Torr for about 15 to 300 seconds. In the exemplary embodiment, no heating or cooling is applied to the oxidation chamber during the NOX process. Oxygen pressure between 110.sup.6 and 1 Torr is preferred for an oxidation time mentioned above in order to achieve a RA in the range of 0.5 to 5 ohm-um.sup.2. A mixture of O.sub.2 with other inert gases such as Ar, Kr, or Xe may also be used for better control of the oxidation process. We have found that the final RA uniformity (1 ) of 0.6 um circular devices is more than 10% when a MgO tunnel barrier layer is rf-sputtered and less than 3% when the MgO tunnel barrier is formed by DC sputtering a Mg layer followed by a NOX process.

(16) Returning to FIG. 1, an important feature of the present invention is the free layer 18 formed on the tunnel barrier layer 17. In a first embodiment illustrated in FIG. 2, the free layer 18 has a laminated structure represented by (CoFe/CoFeB).sub.n, (CoFeM/CoFeB).sub.n, (CoFeLM/CoFeB).sub.n, (CoFe/CoFeBM).sub.n, (CoFeM/CoFeBM).sub.n, or (CoFeLM/CoFeBM).sub.n where n2, L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, and Nb, and M is unequal to L. The CoFeB layer has a [Co.sub.(100-X)Fe.sub.X].sub.100-YB.sub.Y composition where x is from 0 to 100 atomic % and y is between 10 and 40 atomic %. In a CoFeBM alloy, the B content is preferably between 5 and 40 atomic %. In one example, M=Ni to give a CoNiFeB layer. The CoNiFeB layer may be formed by co-sputtering CoB and CoNiFe. CoB is preferably used in the co-sputtering process as a means of adjusting .

(17) In the exemplary embodiment where n=2, CoFe, CoFeM, or CoFeLM layers 18a1, 18a2 alternate with boron containing layers 18b1, 18b2. When M, or L and M are selected from the group of Ta, Ti, W, Zr, Hf, Tb and Nb, the content of M or L+M in the CoFeM alloy and CoFeLM alloy, respectively, is preferably less than 10 atomic %.

(18) A first stack of layers is shown as 18a1/18b1 and a second stack formed on the first stack is designated 18a2/18b2 where layer 18a2 contacts layer 18b1 and layer 18b2 is the uppermost layer. Note that a CoFe layer 18a1 contacts the underlying tunnel barrier layer 17 and thereby separates the CoFeBM or CoFeB layer 18b1 from the tunnel barrier. The present invention also anticipates a free layer structure with a plurality of n stacks of 18a/18b layers wherein n>2 and the two layers in the upper stack would be designated 18an/18bn (not shown) and the 18a and 18b layers are formed in alternating fashion beginning with an 18a1 layer contacting the tunnel barrier layer 17.

(19) Each 18a layer (18a1 to 18an) is comprised of CoFe, CoFeM, or CoFeLM and has a thickness between 2 and 8 Angstroms and each 18b layer (18b1 to 18bn) is comprised of CoFeB or CoFeBM and has a thickness from 10 to 80 Angstroms. Total thickness of free layer 18 is preferably less than 100 Angstroms. Note that the thickness of each of the CoFe, CoFeM, or CoFeLM layers is less than the thickness of each of the CoFeB or CoFeBM layers to maximize the MR ratio. In other words, MR ratio decreases as the CoFe, CoFeM, or CoFeLM content increases in free layer 18. According to the present invention, it is important that an 18a layer contacts the tunnel barrier layer 17. Otherwise, a TMR structure in which a CoFeB or CoFeBM layer 18b contacts a tunnel barrier would lead to a higher noise level and lower MR ratio. It is also important to cap or laminate 18b layers with 18a layers for the purpose of noise reduction.

(20) In a second embodiment, the 18b1, 18b2 layers in an example where n=2 are comprised of CoB and the CoB layer is preferably a low magnetostriction material Co.sub.SB.sub.T where T is from 5 to 30 atomic % and S+T=100 atomic %. Alternatively, one or more CoB layers may be replaced by an alloy such as CoBM or CoBLM. Moreover, one or more of the 18a layers may be comprised of CoFe, a CoFeM alloy, or a CoFeLM alloy as in the first embodiment. When M, or L and M are selected from the group of Ta, Ti, W, Zr, Hf, Tb and Nb, the content of M or L+M in the CoFeM alloy and CoFeLM alloy, respectively, is preferably less than 10 atomic %.

(21) Thus, free layer configurations of the second embodiment are represented by (CoFe/CoB).sub.n, (CoFe/CoBM).sub.n, (CoFe/CoBLM).sub.n, (CoFeM/CoB).sub.n, (CoFeM/CoBM).sub.n, (CoFeM/CoBLM).sub.n, (CoFeLM/CoB).sub.n, (CoFeLM/CoBM).sub.n, and (CoFeLM/CoBLM).sub.n where n2, L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, and Nb, and L is unequal to M. The B content in the CoBM and CoBLM alloy is from about 5 to 30 atomic %.

(22) Each of the 18a layers (CoFe, CoFeM, or CoFeLM) has a thickness from 2 to 8 Angstroms and each of the 18b layers (CoB, CoBM, or CoBLM) has a thickness from 10 to 80 Angstroms. Total thickness of the free layer 18 is preferably less than 100 Angstroms.

(23) Referring to FIG. 3, a third embodiment of the present invention is depicted in which free layer 18 has a configuration represented by (CoFe/CoFeB).sub.n/CoFe, (CoFeM/CoFeB).sub.n/CoFeM, (CoFe/CoFeBM).sub.n/CoFe, (CoFeM/CoFeBM).sub.n/CoFeM, (CoFeLM/CoFeB).sub.n/CoFeLM, or (CoFeLM/CoFeBM).sub.n/CoFeLM where n is 1, and L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, or Nb, and L is unequal to M. As in the first embodiment shown in FIG. 2, the CoFe, CoFeM, or CoFeLM layers 18a alternate with the boron containing layers 18b, and a layer 18a contacts the tunnel barrier layer 17. However, in this embodiment the top layer is a non-boron containing layer. When n=1, a layer 18a1 contacts the tunnel barrier layer, a layer 18b1 is formed on layer 18a1, and layer 18a2 is the uppermost layer in the free layer 18 stack. When n=2, the uppermost layer is layer 18a3 rather than a boron containing layer. As a result, non-boron containing layers (i.e. 18a1, 18a2, 18a3) alternate with boron containing layers (18b1, 18b2). When n>2, the top CoFe, CoFeM or CoFeLM layer may be designated as 18an and the 18a layers from bottom to top are in succession 18a1, 18a2, . . . 18an and alternate with the 18b layers that are formed from bottom to top in succession 18b1, 18b2, . . . 18b(n1). It is also important in this embodiment that the tunnel barrier layer 17 is contacted by an 18a layer and not an 18b layer. Uppermost layer 18an contacts the capping layer (not shown).

(24) In a fourth embodiment, the free layer 18 has a configuration represented by (CoFe/CoB).sub.n/CoFe, (CoFe/CoBM).sub.n/CoFe, (CoFe/CoBLM).sub.n/CoFe, (CoFeM/CoB).sub.n/CoFeM, (CoFeM/CoBM).sub.n/CoFeM, (CoFeM/CoBLM).sub.n/CoFeM, (CoFeLM/CoB).sub.n/CoFeLM, (CoFeLM/CoBM).sub.n/CoFeLM, or (CoFeLM/CoBLM).sub.n/CoFeLM where n1, L and M are one of Ni, Ta, Ti, W, Zr, Hf, Tb, and Nb, and L is unequal to M. The B content in the CoB, CoBM, or CoBLM alloy is from about 5 to 30 atomic %.

(25) In an example where n=1, a layer 18a1 contacts the tunnel barrier layer 17, a layer 18b1 is formed on layer 18a1, and a layer 18a2 is disposed on layer 18b1. When n>1, the top CoFe, CoFeM or CoFeLM layer may be designated as 18an and the 18a layers are formed from bottom to top in succession 18a1, . . . 18an. Likewise, the boron containing 18b layers are formed from bottom to top in succession 18b1, . . . 18b(n1).

(26) After the free layer 18 is formed, a capping layer 19 is deposited on the free layer. Capping layer 19 may be comprised of Ru, Ta, or combinations thereof such as Ru/Ta/Ru. A Ru upper layer is typically preferred since Ru is resistant to oxidation, provides a good electrical connection to an overlying top lead (top shield) formed in a subsequent step, and serves as a CMP stop during a subsequent planarization process.

(27) Once the TMR stack is complete, the partially formed read head 1 may be annealed in a vacuum oven within the range of 240 C. to 340 C. with an applied magnetic field of at least 2000 Oe, and preferably 8000 Oe for about 2 to 10 hours to set the pinned layer and free layer magnetization directions. It should be understood that under certain conditions, depending upon the time and temperature involved in the anneal process, the tunnel barrier layer 17 may become a uniform MgO tunnel barrier layer as unreacted oxygen diffuses into the adjacent Mg layer. In another embodiment, a two step anneal process as described previously may be employed.

(28) Referring to FIG. 4, the TMR stack is patterned by following a conventional process sequence. For example, a photoresist layer 20 may be coated on the capping layer 19. After the photoresist layer 20 is patterned, a reactive ion etch (RIE), ion beam etch (IBE), or the like is used to remove underlying layers in the TMR stack that are exposed by openings in the photoresist layer. The etch process stops on the bottom shield 10 or between the bottom shield and a barrier layer (not shown) to give a TMR sensor with a top surface 19a and sidewalls 21.

(29) Referring to FIG. 5, an insulating layer 22 may be deposited along the sidewalls 21 of the TMR sensor. The photoresist layer 20 is then removed by a lift off process. A top lead otherwise known as a top shield 25 is then deposited on the insulating layer 22 and top surface 19a of the TMR sensor. Similar to the bottom shield 10, the top shield 25 may also be a NiFe layer about 2 microns thick. The TMR read head 1 may be further comprised of a second gap layer (not shown) disposed on the top shield 25.

Comparative Example

(30) An experiment was conducted to demonstrate the improved performance achieved by implementing a free layer in a TMR sensor according to the present invention. A TMR stack of layers, hereafter referred to as Sample A and shown in Table 1, was fabricated as a reference and is comprised of a CoFe/CoB free layer wherein the lower Co.sub.70Fe.sub.30 layer is 5 Angstroms thick and the upper Co.sub.80B.sub.20 layer is 48 Angstroms thick. This free layer was disclosed in related U.S. Pat. No. 8,472,151. All samples in the experiment have a seed/AFM/AP2/Ru/AP1/MgO/free layer/Ru configuration where AP2 and AP1 layers are comprised of CoFe, the seed layer is Ta/Ru, and the AFM layer is IrMn. The MgO tunnel barrier was formed by depositing a 7 Angstrom thick lower Mg layer that was subjected to a NOX process before a 4 Angstrom thick upper Mg layer was deposited. Sample B comprises a free layer represented by (CoFe/CoB).sub.2 according to the second embodiment of the present invention. Sample C (n=1) and Sample D (n=2) have a (CoFe/CoB).sub.n/CoFe free layer according to the fourth embodiment of the present invention.

(31) The thicknesses in Angstroms of the other TMR layers are given in parentheses: Ta(20)/Ru(20) seed layer; IrMn (70) AFM layer; CoFe(25)Ru(7.5)CoFe(20) pinned layer; and Ru(50) capping layer. The TMR stack was formed on a NiFe shield and was annealed with a two step process comprised of a first anneal at 250 C. for 3 hours and a second anneal at 280 C. for 1.5 hours with an applied field of 8000 Oe to achieve high dR/R while maintaining good magnetic softness.

(32) TABLE-US-00001 TABLE 1 Magnetic properties of MgO MTJs with Fe.sub.70Co.sub.30/Co.sub.80B.sub.20 based free layers Sample Free Layer Composition Bs Hc (Oe) Lambda RA dR/R A FeCo5/CoB48 0.60 4.6 1.20 10.sup.6 1.8 60% B FeCo5/CoB24/FeCo5/CoB24 0.68 5.9 8.2 10.sup.7 1.7 60% C FeCo5/CoB50/FeCo5 0.63 5.3 2.0 10.sup.6 1.8 58% D FeCo5/CoB22/FeCo5/CoB22/FeCo5 0.72 6.8 1.3 10.sup.6 1.8 58%

(33) Sample B was formed by inserting a thin CoFe layer in the CoB layer of Sample A and resulted in a slight increase in Hc of about 1 Oe. There was little or no effect on Hc, , RA, and dR/R. For Sample C, a thin CoFe layer was added above the CoB layer in Sample A. Sample D represents insertion of a thin CoFe layer in the CoB layer in Sample A and addition of a thin CoFe layer as the top layer in the free layer stack. A CoFe cap caused a slight decrease of about 2% in dR/R while , Hc, and RA are all comparable to the Sample A reference. All three Samples B-D exhibited reduced noise with no signal loss (not shown) compared with Sample A which is a significant improvement over prior art technology.

(34) Although not bound by theory, we believe that the noise reduction effect may be attributed to a change in free layer microstructure, modification of boron distribution in the CoB layer such that there is a lower concentration near the interface with CoFe layers, magnetic coupling between CoFe layers, or a combination of one or more of the aforementioned effects.

(35) The free layers disclosed in the embodiments found herein may be fabricated without additional cost since no new sputtering targets or sputter chambers are required. Furthermore, a low temperature anneal process may be employed which is compatible with the processes for making GMR sensors. Therefore, there is no change in process flow and related processes compared with current manufacturing schemes.

(36) While this invention has been particularly shown and described with reference to, the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.